Several different types of television broadcast formats and standards are used in different locals throughout the world. Two of the most pervasive are the system used in the United States of America known as NTSC (National Television Standards Committee) and the European system known as Pal (Phase Alternating Line). In addition, several systems for providing increased picture resolution or definition, generally referred to as HDTV (High Definition Television) have been and are continuing to be developed.
While the television receivers operating in these varied systems are equally varied, certain aspects remain generally similar. For example, most television receivers include circuitry for selecting the desired television signal from among a plurality of broadcast signals available, a signal processing system which recovers the picture and sound information from the broadcast signal, systems for sequentially scanning a display device such as a cathode ray tube in both horizontal and vertical directions, and scan synchronization systems operative upon the display to coordinate display scanning to the picture and sound information.
Despite significant differences between the signal selection and signal processing functions of television receivers operating in accordance with the above-mentioned variety of broadcast systems, the functions of display scanning and synchronzation are generally quite similar. Generally, picture and sound inforamtion together with scan synchronizing signals are modulated upon a broadcast carrier at the transmitter. At the receiver, the scan synchronizing signals are separated from the remainder of the picture and sound information and used to control locally generated horizontal and vertical scan signals. The latter are used to drive the scanning circuits of the display system.
Computer monitors and many video game devices are also similar to television receivers in that they include a display system, such as a cathode ray tube, which is scanned in synchronism with picture information. In such system, the scan signals are computer generated and are used to synchronize display scan and picture information in much the same manner as television receivers.
Because the horizontal scan oscillator control used in the variety of television receivers, computer monitors and video game displays are substantially identical in overall function, it seems logical to provide a single system which may be operated for all such uses (e.g. a "universal" system). However, attempts to do so have been frustrated largely by the broad range of scan frequencies over which such a universal scan control system would be required to operate.
As is well known, the operation of display system synchronization controlling the horizontal scan oscillator is extremely demanding. Meeting these demands is made challenging by the need for precise control of both the frequency and phase of the horizontal scan oscillator. Maintaining frequency control over a broad range of reference scan frequencies generally leads to systems which lack stable precise phase control. Conversely, highly precise stable phase locking systems usually lack the ability to accommodate a wide range of frequencies.
For example, one of the most common and pervasive horizontal scan oscillator control systems used in television receivers, computer monitors, or video games is generally referred to as a phase locked loop. While a variety of different phase locked loop systems have been developed, in most, an oscillator produces an output signal having a frequency dependent upon an applied control signal. A phase detector compares the oscillator output signal to the reference signals and produces an error signal indicative of the frequency and phase relationship therebetween. This error signal is amplified to produce a control signal applied to the oscillator in a closed loop arrangement.
As is well known, conventional phase locked loop systems respond to large frequency differences by reaching an equilibrium point in which a sufficient error voltage is maintained by the phase detector to provided the necessary control of the oscillator. This equilibrium results in a condition in which the frequency of oscillator is synchronized to that of the incoming reference sync signals while a phase difference or phase error between oscillator output signals and sync signals remains. This phase error is reffered to in the art as static phase error. The ability of conventional phase locked loop systems to make large frequency compensations is accompanied by correspondingly large static phase errors. Thus, practitioners in the art generally must compromise overall system performance to provide the necessary frequency compensation characteristic at the expense of static phase error.
In attempting to minimize or overcome the need for such compromise of system performance, and thereby accommodate a wider range of scan frequencies, partitioners in the art have endeavored to provide improved more flexible systems. Such attempts have included multiple loop control systems and systems which alter the effective control loop gain in response to frequency lock or out of lock conditions. While such attempts have improved certain aspects of the system performance, they have often been beset by difficulties associated with increased complexity and/or transition difficulties between the in-sync and out-of-sync condition of system operation. In addition, systems attempting to make broad range frequency adjustments may create transition conditions which are capable of damaging the tuned subsystems and related components within the scan and high voltage circuits of the television receiver or monitor. There remains, therefore, a need in the art for an improved horizontal scan oscillator control system which is capable of use in a wide range of scan freqencies while concurrently minimizing the danger of damage to system components during frequency transition.
Accordingly, it is general objects of the present invention to provide an improved horizontal scan oscillator control system operable in response to a broad range of scan frequencies. It is a more particular object of the present invention to provide an improved horizontal oscillator control system which responds to broad frequency signals while avoiding system damage during large frequency transitions.